1. Field of the Invention
The present invention relates to a method and apparatus for measuring the organic carbon content of, for example, ultrapure water, and in particular to a method and apparatus for measuring the organic carbon content of a test liquid by irradiating the test liquid, such as ultrapure water, with ultraviolet light, and measuring the conductivity of the test liquid, which changes due to generated organic acids and carbon dioxide.
This application is based on Japanese Patent Application, No. Hei 11-375977, the contents of which are incorporated here by reference.
2. Description of Related Art
In modern high precision industrial manufacturing processes, for example, highly purified xe2x80x9cultrapure waterxe2x80x9d is frequently used in large amounts. For example, washing semiconductors, production of medicines, injections, and the like, and chemical analysis, pure water that contains substantially no impurities such as particles, various species of ions, microorganisms such as bacteria, or soluble substances such as organic compounds, is indispensable. Systems for producing such pure water normally use a combination of reverse osmosis, distillation, ion exchange, absorption processes, vacuum deaeration, ultraviolet oxidizing, and filtration methods including ultrafiltration. In particular, in the field of semiconductor manufacturing, for example, the gaps between circuits are becoming narrower with the increasing density of LSI integration, and thus in order to prevent circuit shorting, the cleaning water for the semiconductors must be of the highest purity, and not only ions, but to the extent possible, particles, bacteria, and organic materials must be eliminated.
One method of indicating the degree of purity of pure water is the TOC (total organic carbon), which represents the degree of contamination using the amount of carbon in the organic substances in the water. As a method of measuring the TOC value of pure water, the TOC measurement using the ultraviolet (UV) oxidizing method is widely used. In this TOC measurement, a test liquid is introduced into an ultraviolet irradiation part, there the test liquid is irradiated with UV light, and the organic carbon in the test liquid is changed into organic acids and carbon dioxide. Then the TOC value of the test liquid is found based on the change in conductivity of the test liquid obtained thereby.
Several types of TOC measurements for such a UV oxidizing are used. For example, the apparatus disclosed in Japanese Examined Patent Application, Second Publication, No. Sho 63-46375, is known. This apparatus irradiates the ultrapure water test liquid resting in a test cell with UV light, and at the same time measures the change in conductivity during this time interval using a conductivity detecting electrode disposed in the test cell. In addition, upon confirming that the oxidizing reaction due to the UV light has substantially completed by using the change in the rate of conductivity, the organic carbon content is found from the amount of change in conductivity up to this point in time.
In addition, an apparatus is known that provides a measuring flow path that disposes first and second conductivity sensors before and after the UV light irradiating part, causes ultrapure water to flow continuously therethrough at a fixed rate of flow, and measures the organic carbon content based on the difference in the conductivity obtained by the first and second conductivity sensors. This apparatus assumes that if the rate of flow of the ultrapure water flowing through the UV light irradiation part is constant, then the amount of UV light that the ultrapure water receives per unit of volume is constant, which implies a constant degree of progress of the oxidizing reaction. In this case, because measurement is carried out while the oxidizing reaction due to the UV light is not complete and the ultrapure water is continuously flowing, the organic carbon content can be continually measured.
Among the above-described former conventional apparatuses using a method wherein the organic carbon content is found from the amount of change in conductivity up to the substantial completion of the oxidizing reaction due to UV light, the following problems are encountered. First, while the time until the completion of the oxidative reaction depends on the components of the test liquid and the strength of the UV light, about 10 to 20 minutes is necessary. Thus, carrying out monitoring of test liquids in real time is difficult.
In addition, because the test liquid is resting in the test cell until the oxidative reaction is complete, eluate from the materials that form the test cell and the conductivity detecting electrode mix into the test liquid, and the conductivity rises. In contrast, the generated carbon dioxide may leak, causing the conductivity to fall. Therefore, the measured values must be compensated by taking into consideration the changes in conductivity that do not depend on the organic carbon content.
Furthermore, because the state of progress of the oxidative reaction is judged by the change of conductivity, a conductivity detecting electrode must always be disposed in the test cell. Thus, the structure of the test cell tends to become complicated and difficult to manufacture.
In contrast, among the above-described latter conventional apparatuses in which the ultrapure water continuously flows at a constant rate of flow, and the organic carbon content is continuously measured based on the difference in conductivity before and after irradiation by UV light, the following types of problems are encountered. First, as described above, being able to measure without the oxidizing reaction having completed assumes that if the rate of flow of ultrapure water flowing through the UV light irradiation part is constant, then the amount of ultraviolet light received per unit of volume of ultrapure water is constant. If the rate of flow of ultrapure water flowing through the UV irradiation means increases, then the amount of UV irradiation per unit of volume of ultrapure water will decrease, and thus the difference in conductivity will become small. In contrast, if the rate of flow decreases, the amount of UV irradiation per unit of volume of the ultrapure water will increase, and thus the difference in conductivity will become large. This means that in the case that the rate of flow of the ultrapure water flowing through the UV irradiation part changes, a measurement error will occur immediately. In order to avoid this type of error, the flow rate control of the ultrapure water must be carried out with extreme precision. As a result, the liquid conveyance system becomes complicated, and the cost of the system as a whole may become high.
Furthermore, in order to observe the difference in conductivity before and after the UV irradiation, two sensors, i.e., the first and second sensor, and the processing circuits for the signals from these sensors, etc., are necessary. Therefore, in these terms as well, the apparatus becomes complicated and the cost of the system may become high.
In consideration of the above, the present invention has as an object providing a method and apparatus for measurement of the organic carbon content that allows monitoring of the organic carbon amount substantially in real time, and at the same time, does not necessitate precision flow control.
In order to resolve the above-described problems, controlling the amount of UV light impinging on a flowing test liquid by adjusting the time that the UV light source is lit was investigated. The results showed that if the liquid sample passes through the oxidizing processing vessel below a predetermined rate of flow, a portion of test liquid is present that has received the complete irradiation by UV light from the commencement to the extinguishing of the lighting of the ultraviolet light source, and in this case, the amount of irradiated UV light impinging on the test liquid depends on the time that the UV light source is lit.
Specifically, in order to resolve the above-described problems, the present invention provides a measuring method for the organic carbon content that causes the test liquid to flow into the oxidizing process vessel and stops the irradiation after the UV light has irradiated this test liquid for a predetermined time, measures the base conductivity prior to commencement of the lighting of the UV light and the maximum conductivity after irradiation has stopped with a conductivity detecting means provided in proximity to the exit of the oxidizing vessel, and finds the organic carbon content of the test liquid from the difference between this base conductivity and maximum conductivity, wherein the rate of flow F of the test liquid that flows into the oxidizing vessel, the volume of the part of the oxidizing vessel irradiated by the UV light upstream from the conductivity detecting means, and the irradiation time T of the UV light have the relationship Fxe2x89xa6V/T.
Moreover, in the present specification, xe2x80x9cin proximity to the exit of the oxidizing vesselxe2x80x9d, the location where the conductivity direction means is disposed, is meant to include both the inside of the oxidizing process vessel upstream from the exit of the oxidizing process vessel and the outside of the oxidizing process vessel downstream from the same. In addition, in the case of the inside of the oxidizing process vessel, both the area in the part irradiated by the UV light and the area outside the irradiated part are included.
In addition, in the present specification, the xe2x80x9cUV light irradiation time Txe2x80x9d is not only the time that the complete and continuous irradiation lasts, but includes the time that the irradiation from the light source (for example, a xenon flash lamp) is lit at a constant frequency. This means that the separate flashes during irradiation time T do not correspond to being lit or extinguished or being turned on and turned off.
According to the present invention, portion of test liquid is present that has received the complete irradiation from the commencement to the extinguishing of the lighting of the UV light source, and this shows the maximum conductivity based on the degree of the progress of the oxidizing reaction that depends on the UV irradiation time, and thus the organic carbon content can be found irrespective of fluctuations in the rate of flow.
After this maximum conductivity is measured, preferably the test liquid in the oxidizing vessel is exchanged by increasing the rate of flow at which the test liquid passes through the oxidizing vessel. Thereby, the test liquid that includes oxidized products that remain in the oxidizing vessel can be expelled in a short period of time, and thus the time until the next measurement can be shortened. In addition, even if bubbles are produced in the oxidizing vessel and the conductivity detecting means, when the rate of flow is increased, they can be caused to flow out and be eliminated.
In addition, in order to promote the UV oxidizing of the organic carbon in the test liquid, preferably a photocatalyst is used. Thereby, even with an identical UV light irradiation time, larger fluctuations in conductivity can be obtained, and thus the detection sensitivity can be improved.
In addition, in the case that the amount of UV light is measured and the amount of the measured light is smaller than a predetermined value, a warning can be output. Thereby, the user can be notified about the deterioration of the light source.
In addition, the present invention provides a measuring apparatus for organic carbon content that is characterized in providing an oxidizing process vessel through which the test liquid passes, a UV light source that irradiates the test liquid in the oxidizing process vessel with UV light, a light control means that turns off the UV light source after being lit for a predetermined time, a conductivity detecting means that is provided in proximity to the exit of the oxidizing process vessel, and a calculating means that calculates the organic carbon content in the test liquid from the difference between a base conductivity before commencement of the lighting of the UV light and a maximum conductivity after turning off the UV light source that is measured by this conductivity detecting means, and at the same time provides a flow rate control means that controls the rate of flow F such that the rate of flow F at which the test liquid passes through the oxidizing process vessel, the volume V of the part of the oxidizing process vessel irradiated by the UV light that is upstream from the conductivity detecting means, and the irradiation time of the UV light have the relationship Fxe2x89xa6V/T.
According to the present invention, portion of test liquid is present that has received the complete irradiation from the commencement to the extinguishing of the lighting of the ultraviolet light source, and in the conductivity detecting means, a maximum conductivity based on the degree of progress of the oxidizing reaction that depends on the UV light irradiation time interval can be obtained. Thereby, the amount of the organic carbon can be found irrespective of fluctuations of the rate of flow.
As explained above, a means that exchanges the lest liquid in the oxidizing process vessel by increasing the rate of flow of the test liquid passing through the oxidizing process vessel after the maximum conductivity is measured is desirable. In addition, in order to promote the UV oxidizing of the organic carbon in the test liquid, preferably a photocatalyst is provided in the oxidizing process vessel.
In the case that a photocatalyst is provided in the oxidizing process vessel, the oxidizing process vessel is a two-layer pipe structure in which the test liquid passes through the oxidizing vessel between an inner tube comprising a material that substantially transmits UV light and an outer tube, the inside of the outer tube is covered by a photocatalyst, and the UV light source is disposed on the inner tube side. In this case, the UV light source can be accommodated inside the tube, or the outer tube of the UV light source can also act as the inner tube of the oxidizing process vessel.
In addition, preferably the apparatus of the present invention provides a photometer that measures the amount of UV light from the UV light source. Thereby, in the case that the amount of light measured by the photometer falls below a predetermined value, a warning, for example, can be output that notifies the user that it is time to exchange the UV light source. In addition, compensation of the organic carbon content that takes into account the fluctuating and decreasing amount of UV light can also be considered.
Furthermore, the apparatus of the present invention preferably has a means to confirm the rate of flow F of the test liquid that is passing through the oxidizing process vessel. Thereby, in the case that the rate of flow F, the volume V of the part of the oxidizing process vessel that is irradiated by UV light upstream from the conductivity detecting means, and the irradiation time T of the UV light do not maintain the relationship Fxe2x89xa6V/T for any reason, a warning can be issued, for example.
According to the present invention, measurement of the organic carbon content can be performed in the extremely short interval of once every a few minutes. Thus, monitoring of the organic carbon content can be carried out substantially in real time. Furthermore, because precision control of the rate of flow is not necessary in order to obtain the measured values, an apparatus having a simple structure is possible. Therefore, an apparatus for measuring the organic carbon content that is low cost and easy to use can be provided.